The present invention relates to a steering rack bar which is one of the components of steering equipment for automobiles, and also relates to its manufacturing method. Recently, for the purpose of weight reduction in automobiles, it is tried to make such as shafts hollowed which has been usually solid. The object of this invention is a hollow steering rack bar for such purpose. The present invention provides hollow steering rack bars of excellent quality and enough strength in low costs.
The steering rack bar has a shape that a rack is formed, for instance, along about half-length of a shaft with a circular crosssection. For making hollowed for weight reduction, conventionally a solid bar has been drilled a hole after cutting a rack on the surface. It can be considered that the rack is cut on a flattened part of a tube by making use of an originally hollow material, namely, the tube. However, actually it is difficult to secure necessary strength of the rack, because the thickness at the part of the rack becomes very thin by the cutting. On the other hand, it was proposed that only the part where a rack is not formed is made hollowed, as abandoning to make hollowed the part where the rack is formed. Japanese laying-open patent publication 9-58489 concerns the above technique, wherein a rack bar is completed by crimping to join a steel tube and a solid bar which is formed a rack.
Another method is proposed in Japanese patent publication 3-5892, wherein a rack is formed by plastic working on a raw steel tube, so as to make a steering rack bar which is hollowed in the overall length. Several improvements to the above method have been devised, then, production by the above method is rapidly increasing. The method disclosed in Japanese patent publication 3-5892 consists of two processing stages, i.e. preliminarily flattening of a part of the steel tube, and then forming a rack on the flattened part. In the first place, a part of a raw steel tube to be processed is held in a primary-forming split dies, which can be opened right and left, and has a penetrating hole in a part to be formed a rack at a state of the dies being closed. Then, a punch having a flat top is inserted into the hole and the part of steel tube is flattened, for making a primary formed piece. In the next stage, the primary formed piece is held in secondary-forming split dies which can be opened right and left and has an inner shape coinciding with the outer shape of the completed rack bar. Namely, the secondary forming split dies have a female pattern of rack teeth in their upper parts at a state of the dies being closed. Then, mandrels are inserted into the steel tube in order to perform ironing of the previously flattened part from the inside of the steel tube. Consequently, the outer surface of the primarily formed piece is bulged, and a rack is formed according to the shape of the secondary-forming split dies.